Fix host cleanup terminating CI runners

[superposition]

Summary

• send SIGINT only to the configured host so Topcoat can run its own child cleanup
• keep the isolated process-group kill as a five-second fallback
• add a Unix regression test that verifies Coatcheck's parent survives
• prepare patch release 0.2.1

Validation

• cargo fmt --all -- --check
• cargo test --all-targets --all-features (13 passed)
• cargo clippy --all-targets --all-features -- -D warnings
• cargo package --locked (24 files, 119.6 KiB, verified)